drm: Zero out DRM object memory upon cleanup

Drivers where the DRM objects have a lifetime that extends beyond that
of the DRM device need to zero out the DRM object memory to void stale
data such as properties. The DRM core code expects to operate on newly
allocated and zeroed out objects and will behave unexpectedly, such as
add duplicate properties, otherwise.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>